To the Editor.—In its analysis of the risk of invasive group A streptococcal infection among household contacts of case patients, the Working Group on Prevention of Invasive Group A Streptococcal Infections1 used data derived from surveillance conducted in Ontario to conclude that the rate of infection in household contacts exceeds the overall rate in the population by almost 200-fold. This computation is based on comparing the incidence of cases among contacts with the annual incidence in Ontario. The observation period for incidence among contacts, however, is only 1 month. To compute the relative risk for the contacts, the background incidence likewise should be expressed as a monthly incidence. If this is done by dividing the annual rate by 12, the rates per month are 294 cases per 100,000 contacts and 0.125 case per 100,000 in the overall population. Recalculating excess risk with these rates, the excess risk in household contacts is more than 2300-fold. This approach to calculation does not take into account effects of seasonal clustering of cases. Nevertheless, it should provide a more realistic estimate of increased risk than the approach used by the Working Group.